debian: remove ubuntu/xenial patch (distro not supported any more)

As zstd is too old in ubuntu/xenial (0.5.1 while min version is 0.8.1), this
distro is not supported any more.

tools/debian/patches/weechat_ubuntu_xenial.patch

@@ -1,62 +0,0 @@
-diff --git a/debian-devel/compat b/debian-devel/compat
-index 48082f72f..ec635144f 100644
---- a/debian-devel/compat
-+++ b/debian-devel/compat
-@@ -1 +1 @@
--12
-+9
-diff --git a/debian-devel/control b/debian-devel/control
-index bf94a36f1..1eb804643 100644
---- a/debian-devel/control
-+++ b/debian-devel/control
-@@ -5,7 +5,7 @@ Priority: optional
- Build-Depends:
-     asciidoctor (>= 1.5.4),
-     ruby-pygments.rb,
--    debhelper (>= 12),
-+    debhelper (>= 9),
-     cmake, pkg-config,
-     libncursesw5-dev,
-     gem2deb,
-@@ -13,8 +13,8 @@ Build-Depends:
-     libaspell-dev,
-     liblua5.3-dev,
-     tcl8.6-dev,
--    guile-2.2-dev,
--    php-dev, libphp-embed, libargon2-dev, libsodium-dev,
-+    guile-2.0-dev,
-+    php-dev, libphp-embed,
-     libxml2-dev,
-     libcurl4-gnutls-dev,
-     libgcrypt20-dev,
-diff --git a/debian-stable/compat b/debian-stable/compat
-index 48082f72f..ec635144f 100644
---- a/debian-stable/compat
-+++ b/debian-stable/compat
-@@ -1 +1 @@
--12
-+9
-diff --git a/debian-stable/control b/debian-stable/control
-index ccf81cac8..7098b52c6 100644
---- a/debian-stable/control
-+++ b/debian-stable/control
-@@ -5,7 +5,7 @@ Priority: optional
- Build-Depends:
-     asciidoctor (>= 1.5.4),
-     ruby-pygments.rb,
--    debhelper (>= 12),
-+    debhelper (>= 9),
-     cmake, pkg-config,
-     libncursesw5-dev,
-     gem2deb,
-@@ -13,8 +13,8 @@ Build-Depends:
-     libaspell-dev,
-     liblua5.3-dev,
-     tcl8.6-dev,
--    guile-2.2-dev,
--    php-dev, libphp-embed, libargon2-dev, libsodium-dev,
-+    guile-2.0-dev,
-+    php-dev, libphp-embed,
-     libxml2-dev,
-     libcurl4-gnutls-dev,
-     libgcrypt20-dev,